Design Of Pressure Hulls Research Articles

The relevance of analytical formulations predicting stiffener tripping

In search for the governing failure mode of externally pressurized ring-stiffened cylinders the focus is, due to its loading and slender geometry, drawn to elastic buckling. However, the hydrostatic pressure loading and imperfections cause nonlinear behaviour, and excessive deformation results in plastic failure, denoted by collapse. This implies that a true bifurcation point is often hardly observed, and this raises the question whether it is worthwhile to spend much effort to determine the elastic buckling pressures other than to ensure they are well above the collapse pressure? This paper focuses on the elastic buckling of ring frames, generally referred to as frame tripping. Some latest attempts in improving the accuracy of the analytical formulations will be questioned. Firstly, this study will compare non-linear buckling with linear-elastic material behaviour to elastic buckling by means of finite element analysis. Secondly, the non-linear elastic-plastic effects will be included too. Doing so, the effect of the geometrical non-linearity and material elastic-plastic non-linearity will be visible separately and will give an indication of the relevance of frame tripping results predicted by analytical elastic buckling formulations. Literature offers a variety on analytical formulations, each with an underlying idea to remove undesired assumptions that impair results. Even recently comprehensive formulations are published to establish an accurate value of the elastic tripping pressures. This paper shows whether those methods are still relevant in modern pressure hull design or not.

Design method of combined stiffened pressure hulls with variable-curvature characteristic

This study presents a combined pressure hull design method with variable-section ribs. This method for determining the length-radius ratio of the pressure hull's bearing unit is derived based on the stiffness-matching characteristic of the shell and stiffener. The bearing mechanism of variable curvature stiffened shell is discussed through theoretical derivation and numerical simulation. The design scheme of variable cross-section and the equal-stiffness rib is proposed to address the limitations of outer envelope size and inner space utilization of the pressure hull. Its buckling load is determined by the energy method. The design approach for a new type of stiffened pressure hull is summarized. In addition, the method is verified by the design example and the experiment. The results revealed that the combined variable curvature shells are a superior pressure hull configuration. Compared to the cylindrical stiffened shell, the buckling load of the pressure hull with variable cross-section ribs can be improved significantly (about 26.7%). At the same time, space utilization remains at the original level. The stiffened design technique described in this research based on elastic buckling theory can accurately determine the length-radius ratio of the bearing unit, relying on the design objective with high feasibility.

APPLYING MODERN PROCESS AUTOMATION AND OPTIMISATION TOOLS TO THE DESIGN OF SUBMARINE STRUCTURES

This paper describes the application of commercially available process automation and optimisation software to the design of submarine pressure hulls. The Isight software provided by Dassault Systemes, which integrates with a range of internal and external tools, including their Abaqus FE software, and provides access to numerous sophisticated design assessment and optimisation routines, was used. Within the Isight environment, disparate tools can be linked in structured workflows and the following case studies are described: • Minimum weight pressure hull design for different operating depths using traditional design formulae. • Design refinement for weight reduction using linear elastic FE. • Sensitivity of pressure hull collapse strength to shape imperfection using elasto-plastic non-linear FE. Automating these processes does lead to an increase in the amount of output, which can require an experienced eye to interpret, but they have produced demonstrable design improvements that just would not have been realised by manual methods.

Pressure Hull Design Methods for Unmanned Underwater Vehicles

This paper describes design methods for the plastic hull of an Unmanned Underwater Vehicle (UUV), with a particular focus on its cylindrical body and nearly spherical domes at the ends. With the proposed approach, the methodologies reported in the literature were compared, and suitable modifications and improvements were investigated and implemented to extend the classical theories and data to this case study. The investigated underwater vehicle, named FeelHippo, was designed and assembled by the Department of Industrial Engineering of the University of Florence. Its main hull is composed of an extruded PMMA (PolyMethyl MethAcrylate) cylinder and two thermoformed PMMA domes. Breakage of the hull results in destructive phenomena, namely, yielding and buckling. An experimental campaign and FEM (Finite Element Method) analysis were carried out to complete the theoretical study, and the collapse pressures were compared with the derived design values. In conclusion, the proposed innovative method is a lean and effective technique for designing underwater hull domes and predicting the collapse pressures.

Numerical Analysis of Sandwich Composite Deep Submarine Pressure Hull Considering Failure Criteria

The pressure hull is the primary element of submarine, which withstands diving pressure and provides essential capacity for electronic systems and buoyancy. This study presents a numerical analysis and design optimization of sandwich composite deep submarine pressure hull using finite element modeling technique. This study aims to minimize buoyancy factor and maximize deck area and buckling strength factors. The collapse depth is taken as a base in the pressure hull design. The pressure hull has been analyzed using two composite materials, T700/Epoxy and B(4)5505/Epoxy, to form the upper and lower faces of the sandwich composite deep submarine pressure hull. The laminated control surface is optimized for the first ply failure index (FI) considering both Tsai–Wu and maximum stress failure criteria. The results obtained emphasize an important fact that the presence of core layer in sandwich composite pressure hull is not always more efficient. The use of sandwich in the design of composite deep submarine pressure hull at extreme depths is not a safe option. Additionally, the core thickness plays a minor role in the design of composite deep submarine pressure hull. The outcome of an optimization at extreme depths illustrates that the upper and lower faces become thicker and the core thickness becomes thinner. However, at shallow-to-moderate depths, it is recommended to use sandwich composite with a thick core to resist the shell buckling of composite submarine pressure hull.

Structural-Acoustic Modeling and Optimization of a Submarine Pressure Hull

<div class="section abstract"><div class="htmlview paragraph">The Energy Finite Element Analysis (EFEA) has been validated in the past through comparison with test data for computing the structural vibration and the radiated noise for Naval systems in the mid to high frequency range. A main benefit of the method is that it enables fast computations for full scale models. This capability is exploited by using the EFEA for a submarine pressure hull design optimization study. A generic but representative pressure hull is considered. Design variables associated with the dimensions of the king frames, the thickness of the pressure hull in the vicinity of the excitation (the latter is considered to be applied on the king frames of the machinery room), the dimensions of the frames, and the damping applied on the hull are adjusted during the optimization process in order to minimize the radiated noise in the frequency range from 1,000Hz to 16,000Hz. Constraints on the total amount of damping that can be used are considered (resource driven constraints) and structural collapse constraints are also taken into account in order to avoid degrading the structural integrity of the pressure hull. Two different optimization strategies are exercised. First a concurrent multidisciplinary analysis is performed; optimal configurations for structural performance and for acoustic radiation are identified and the results are used for producing a single design with optimized performance in both disciplines. Then, an analysis based on set-based design principles is performed. The latter identifies several alternative and diverse hull configurations that provide similar levels of performance with respect to the radiated noise. Having several alternative solutions of nearly equal performance provides insight into the design trade-offs when configuring the pressure hull. The results from both optimization strategies are analyzed and discussed.</div></div>

Experimental and numerical studies on the buckling of the hemispherical shells made of maraging steel subjected to extremely high external pressure

This work is devoted to the nonlinear buckling of the hemispherical shells subjected to extremely high external pressure. The shells are made of maraging steel with yield strength of 2100 MPa. The experiment on a tentatively devised laboratory-scale hemispherical shell is conducted in high pressure chamber, after preciously fabricated, optically and ultrasonically measured. Numerical predictions are carried out and proved to have a good agreement with experimental results. The laboratory-scale hemispherical shell can borne the pressure as high as about 139 MPa and demonstrates an unstable post-buckling character and a local dimple post-buckling mode near the base. Based on the study, a segmented hemispherical configuration is conceived and evaluated numerically, which can provide a reference for the deep-sea pressure hull design made of maraging steel.

수압을 받는 원통형 실린더의 초기부정을 고려한 좌굴해석

Pressure hulls of submerged structures are generally designed as circular cylinders, spheres or cones with form of axisymmetric shell of revolution to withstand the high external pressure of deep ocean. The compressive buckling (implosion) due to hydrostatic pressure is the main concern of structural design of pressure hull and many design codes are provided for it. It is well-known that the buckling behavior of thin shell of revolution is very sensitive to the initial geometric imperfections introduced during the construction process of cutting and welding. Hence, the theoretical solutions for thin shells with perfect geometry often provide much higher buckling pressures than the measured data in tests or real structures and more precise structural analysis techniques are prerequisite for the safe design of pressure hulls. So this paper dealt with various buckling pressure estimation techniques for unstiffened circular cylinder under hydrostatic pressure conditions. The empirical design equations, eigenvalue analysis technique for critical pressure and collapse behaviors of thin cylindrical shells by the incremental nonlinear FE analysis were applied. Finally all the obtained results were compared with those of the pressure chamber test for the aluminium models. The pros and cons of each techniques were discussed and the most rational approach for the implosion of circular cylinder was recommended.

CAE 기법을 활용한 심해 내압구조물의 최적설계에 관한 연구

Geometric configurations such as hull shape, wall thickness, stiffener layout, and type of construction materials are the key factors influencing the structural performance of pressure hulls. Traditional theoretical approach provides quick and acceptable solutions for the design of pressure hulls within specific geometric configuration and material. In this paper, alternative approaches that can be used to obtain optimal geometric shape, wall thickness, construction material configuration and stiffener layout of a pressure hull are presented. CAE(Computer Aided Engineering) based design optimization tools are utilized in order to obtain the required structural responses and optimal design parameters. Optimal elliptical meridional profile is determined for a cylindrical pressure hull design using metamodel-based optimization technique implemented in a fully-integrated parametric modeler-CAE platform in ANSYS. While the optimal composite laminate layup and the design of ring stiffener for a thin-walled pressure hull are obtained using gradient-based optimization method in OptiStruct. It is noted that the proposed alternative approaches are potentially effective for pressure hull design.

Minimum weight design of submersible pressure hull under hydrostatic pressure

In this study, we proposed a minimum weight design of a submarine pressure hull under hydrostatic pressure with constraints on factors such as general instability, buckling of shell between frames, plate yielding, and frame yielding. A typical submarine pressure hull is also adopted for a prototype model. This design problem is not only formulated as a discrete nonlinear, nondifferentiable, multimodal constrained minimization problem, but also solved by using the backtrack programming method. Results in this study indicate that the solution for the optimal model's weight reduces an average 6.65% more than the prototype model. The process in this study is favorable for the submersible pressure hull design process.

A Novel Submarine Pressure Hull Design

The paper presents an alternative design for a submarine pressure hull, which has no ring-stiffeners, but where shell instability and general instability are resisted by making the pressure hull of a swedged-shaped form. Calculations on two different-sized pressure hulls appear to indicate that the swedged-shaped hulls are more structurally efficient than their ring-stiffened equivalents.

Pressure Hull Optimization Using General Instability Equation Admitting More Than One Longitudinal Buckling Half-Wave

Current design procedures often assume that the critical buckling shape in general instability contains only one longitudinal half-wave. This assumption may be incorrect for optimal hulls if what appears to be a questionable independent frame buckling constraint, often employed in pressure hull design, is relaxed. A method for predicting general instability admitting more than one longitudinal l half-wave is, therefore, adapted to an automated optimal design procedure for the synthesis of frame-stiffened, cylindrical, submersible hulls. Design studies demonstrate that substantial weight reductions are possible for hulls designed for conventional operating depths (around 1000 ft) if ultrahigh-strength steels are utilized.